Punnett Squares

4. Probability and Inheritance

Probability and Inheritance

The same rules of probability in coin tossing apply to the main events that determine the genotypes of offspring. These events are the formation of gametes during meiosis and the union of gametes during fertilization.

Probability and Gamete Formation

How is gamete formation like tossing a coin? Consider Mendel’s purple-flowered pea plants again. Assume that a plant is heterozygous for the flower-color allele, so it has the genotype Bb (see Figure below). During meiosis, homologous chromosomes—and the alleles they carry—segregate and go to different gametes. Therefore, when the Bb pea plant forms gametes, the B and b alleles segregate and go to different gametes. As a result, half the gametes produced by the Bb parent will have the B allele and half will have the b allele. Based on the rules of probability, any given gamete of this parent has a 50 percent chance of having the B allele and a 50 percent chance of having the b allele.

Probability and Fertilization

Which of these gametes joins in fertilization with the gamete of another parent plant? This is a matter of chance, like tossing a coin. Thus, we can assume that either type of gamete—one with the B allele or one with the b allele—has an equal chance of uniting with any of the gametes produced by the other parent. Now assume that the other parent is also Bb. If gametes of two Bb parents unite, what is the chance of the offspring having one of each allele like the parents (Bb)? What is the chance of them having a different combination of alleles than the parents (either BB or bb)? To answer these questions, geneticists use a simple tool called a Punnett square.

CK-12 Foundation, Biology. http://creativecommons.org/licenses/by-nc-sa/3.0/